Phthiraptera

Parasitic lice

This tree diagram shows the relationships between several groups of organisms.

The root of the current tree connects the organisms featured in this tree to their containing group and the rest of the Tree of Life. The basal branching point in the tree represents the ancestor of the other groups in the tree. This ancestor diversified over time into several descendent subgroups, which are represented as internal nodes and terminal taxa to the right.

You can click on the root to travel down the Tree of Life all the way to the root of all Life, and you can click on the names of descendent subgroups to travel up the Tree of Life all the way to individual species.

Introduction

Lice belong to the order Phthiraptera, and are the only truly parasitic group amongst the exopterygote insects. As permanent ectoparasites of most birds and mammals they exhibit a remarkable level of host specificity which is unparalleled in most other metazoan parasites. Most individuals will spend their entire life cycle on a single host, with transmission largely occurring opportunistically when hosts are in close contact with each other, such as during breeding (but see also phoresy). This unique lifestyle has led to numerous adaptations according to their precise ecological niche on the host, consequently lice are considerably diverse in terms of their size and general body form. Specializations in the diet of lice underpin their major taxonomic divisions and they can be broadly separated into those that feed on skin debris, feathers and fur, and those that have specialized in blood feeding.

To date there are more than 3000 known species of lice and yet many more remain undescribed. With the possible exception of those species that impinge on the activity of humans and their livestock, the true biology of this cryptic group of insects remains obscure, but their remarkable host specificity has attracted much recent attention by evolutionary biologists interested in the ecology and cospeciation of lice and their hosts.

Characteristics

Wingless (apterous) ectoparasites

Three nymphal instars

Hemimetabolous (having a simple metamorphosis, i.e. no pupa)

Mouthparts - mandibles for chewing lice, stylets for sucking lice

Tarsi 1 or 2 segmented (1 in Anoplurans)

Dorsoventally flattened head

Development of operculum in egg

Reduced labial palps

Reduced compound eyes and no ocelli

Antennae are 3 to 5 segmented and capitate

Recessed into the head in the Amblycera

Filiform in the Ischnocera (May be modified as clasping organs in the male)

Typically short in Anoplura

Classification

The order Phthiraptera has been traditionally divided into two groups according to their different feeding habits: the chewing lice or "Mallophaga", and the Anoplura, colloquially known as the sucking lice. It is commonly assumed that the order is derived from a primitive Pscopteran-like ancestor which became parasitic first on birds.

Chewing lice with their large head and mandibles comprise the largest group with some 2900 species. These are separable into three distinct superfamilies - the Amblycera, Ischnocera and Rhyncophthirina. Amblycerans are found on a diverse range of mammals and birds, and have retained some of the more primitive characteristics of the order, with a lesser degree of specialization for particular habitats. This is reflected in their classification, and they divided into around sixty homogenous genera, whilst the Ischnocera are contained by over a hundred and fifty genera, of which around three quarters are confined to birds. Amblycera chew away at younger feathers and soft areas of the skin, causing localized bleeding from which some can drink. They usually roam freely about the surface of their host and seldom attach firmly to fur or feathers, unlike the avian Ischnocera which are more site specific, and will attach securely to the feathers or fur to escape the preening activity of their host. Ischnocera confine themselves to feeding on the downy part of feathers and softer fur. The Rhyncophthirina comprises a single genus which parasitise elephants and wart-hogs. They are a small louse carrying mandibles at the end of a long proboscis-like snout. This allows them to drill through their hosts thick skin.

Anoplura are a much smaller group comprised of some 500 species. These are restricted to mammals, and like the Rhyncophthirina, feed using maxillae positioned at the end of a snout-like protrusion to pierce the skin. Feeding solely on blood they remain at the feeding site causing localized skin irritations to their host. Because of this they are the vectors to a number of blood borne diseases. This group includes the human louse Pediculus humanus, consequently they are probably the most well studied louse group.

Discussion of Phylogenetic Relationships

The phylogenetic relationships and classification of the four main groups of lice have been matters of contention for some time. A cladistic analysis of the morphological data conducted by Lyal (1985) detailed objections to the traditional classification of lice into "Mallophaga" and Anoplura. His study supported the monophyly of the Phthiraptera, with Anoplura and Rhyncophthirina forming a monophletic group, sister group to the Ischnocera; the Amblycera are the sister group of this assemblage. This working hypothesis seems to have received general support amongst most leading lice systematists. Nevertheless, the familial relationships amongst most major groups of lice are poorly understood, and resolution of these relationships awaits an analysis of all available morphological data, as well as the gathering of molecular information. These relationships will not be easy to uncover as many groups have had a long and complex independent history, frequently involving instances of parallel and convergent evolution.

Other Names for Phthiraptera

Vernacular Names:
Parasitic lice

References

Clay, T. 1949. Some problems in the evolution of a group of ectoparasites. Evolution 3: 279-299.

Clay, T. 1951. An introduction to a classification of the avian Ischnocera (Mallophaga): Part I. Transactions of the Royal Entomological Society of London 102,2:171-195.

Clay, T. 1970. The Amblycera (Phthiraptera: Insecta). Bulletin of the British Museum of Natural History Entomology 25: 75-98.

Durden, L. A. and G. G. Musser 1994. The sucking lice (Insecta, Anoplura) of the world: A taxonomic checklist with records of mammalian hosts and geographic distributions. Bulletin of the British Museum of Natural History 218: 90 pp.

Hafner, M. S. and R. D. M. Page 1995. Molecular phylogenies and host-parasite cospeciation: gophers and lice as a model system. Philosophical Transactions of the Royal Society of London. Series B. 349: 77-83.

Hopkins, G. H. E. and T. Clay 1952. A checklist of the genera and species of mallophaga. London, British Museum of Natural History.

About This Page

The initial Tree-of-Life page for the Phthiraptera was prepared by V.S.
Smith and R.D.M. Page with the assistance of Bill Young. Feedback from readers regarding
errors, mistakes and ommissions will be appreciated as will suggestions for
and contributions of photographs.

Each ToL branch page provides a synopsis of the characteristics of
a group of organisms representing a branch of the Tree of Life. The
major distinction between a branch and a leaf of
the Tree of Life is that each branch can be further subdivided into
descendent branches, that is, subgroups representing distinct genetic
lineages.